This research is concerned with the assembly pathway for bacteriophage lambda heads, with particular emphasis on protein processing reactions. Two such reactions have been defined, one a proteolytic cleavage of a head protein and the other a covalent fusion and proteolytic cleavage of two distinct proteins. Proposed experiments are concerned with the chemical nature of these reactions, the locations of the processed proteins in the head structure, and the role of processing reactions in the assembly process. Experiments are also planned dealing with the role of the host in assembly and its relation to protein processing, the role of proteins transiently involved in assembly, and the nature of early intermediate structures of the assembly pathway.